Don't worry Turupawn, I fixed it already. Here's the full step on how I did it:1.) Remove the BG from your map folder. Set it where you can find it (you going to put it back later) 2.) Open all of your osu files.3.) Remove the 0,0,"BG.jpg" from all of your osu files:[Events]
//Background and Video events
0,0,"BG.jpg" <---------------------------------------------------------------- REMOVE THIS PART.
Video,-300,"Yasashisa no Riyuu (TV Size).avi"
//Break Periods
//Storyboard Layer 0 (Background)
//Storyboard Layer 1 (Fail)
//Storyboard Layer 2 (Pass)
//Storyboard Layer 3 (Foreground)
//Storyboard Sound Samples
Sample,3966,0,"soft-hitclap.wav",40
Sample,15831,0,"soft-hitclap.wav",70
Sample,79390,0,"soft-hitclap.wav",60
//Background Colour Transformations
3,100,163,162,255
4.) Delete the osb file from your map if your map does NOT have a storyboard. If your map does have a storyboard, set the osb file aside (you will put it back later). If your storyboard, however, is scripted by hand, skip this step and move on to the next step.5.) Open osu! (if you haven't already) and go to the osu! editor. If you already at the osu! editor (when you did the prevoius steps), simply just go to File > Revert to saved and exit the editor (so that osu! can refresh it completely).6.) Open the editor again and full submit the map.7.) Wait at least 24-48 hours for the old BG in the thumbnail disappears.8.) Find your BG (the one I told you to remove and set aside at step #1) and change the name of the BG to something else (i.e. "Background"). Also, don't forget to put back the osb file back as well (if it has your storyboard of course).9.) Full Submit the map again.10.) Wait at least 24-48 hours. For some reason, it takes about 10 hours (at least for me), but it may vary for you.11.) If done right, your new BG should magically appear at your thumbnail.So far, this trick have never failed. Thanks for helping me though! 
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